Subjects at risk for AIDS, narcotic addicts and homosexuals develop a new syndrome of immunologic thrombocytopenic purpura (ITP) which has become part of the AIDS-related complex (ARC). This syndrome is associated with markedly elevated platelet bound IgG and C3C4, circulating immune complexes, and anti-F(ab')2 antibodies, suggesting an immune complex etiology. Narcotic addict patients, unlike homosexual patients, also have specific anti-platelet 7S IgG antibody. Unlike the situation with classic autoimmune thrombocytopenic purpura, there is no inverse correlation between platelet count and platelet-bound IgG. We propose to compare thrombocytopenic narcotic addicts (NITP) and homosexuals (HSITP) in order to: 1) Investigate the rate of conversion to AIDS over time, using a constant-risk model. Will all patients develop AIDS? Will there be a difference between NITP and HSITP patients? 2) Investigate the ability to culture virus from lymphocytes and semen in the presence and absence of plasma. This would determine a) whether ability to culture virus is related to resistance to disease; b) whether plasma neutralizing antibodies inhibit ability to culture virus and affect resistance to disease; c) relative infectivity of the narcotic addict group who are at risk for spread of virus to the heterosexual community. 3) Determine whether the lack of inverse correlation between platelet count and platelet-bound IgG in NITP and HSITP patients is due to reticulo-endothelial blockade. 4) Determine whether anti-F(ab')2 antibodies in NITP and HSITP patients are anti-idiotypic against anti-viral neutralizing antibodies. Can these regulate the patient's ability to handle HTLV-III infection? 5) Determine whether HTLV-III is present on platelets or within circulating immune complexes, to study the mechanism of the thrombocytopenia. 6) Determine antigenic determinants on platelets against which specific 7S autoantibody is directed in NITP patients.